Molecular Imaging is emerging as a powerful and sensitive technique for functional imaging of the biological functions of the human body. In particular Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET) are two of the most effective technologies for the detection and staging of cancer.
In these techniques, tracers comprising compounds—such as simple sugars—labeled with radiation-emitting radio-pharmaceutical compounds are injected into the patient. The radiation gives rise to gamma-rays that are detected and recorded as the tracer travels through the body and collect in the organs targeted for examination. Cancer cells metabolise sugar at higher rates than normal cells, and the radio-pharmaceutical is drawn in higher concentrations to cancerous areas.
Gamma-ray detectors are used to detect radiation emitted from within the patient. The radiation is created when isotropically emitted positrons slow down and interact with electrons in the tissue and are annihilated. The annihilation of electron and positron produces two 511 keV gamma-rays that are emitted in opposite directions, that is, directions essentially 180° apart. The gamma-ray photons are detected in coincidence using opposing detectors.
When two gamma-rays of the correct energy are detected in coincidence and identified, computed tomography algorithms are used to reassemble each of these valid events into images. In order validly to detect a coincidence event, however, the opposing 511 keV gamma-ray must: (1) be detected in opposing detectors within a narrow time window, Δt; and (2) must have the correct energy, that is 511 keV±ΔE, where ΔE is typically 10% or ˜50 keV. Higher numbers of false coincidences are detected if Δt and/or ΔE is increased, but excessively protracted measurement time (or tracer dose) becomes necessary if count rate is too small, owing to excessively small Δt or ΔE.
The gamma-ray detectors used in PET machines have a finite response time to incoming radiation events. Owing to the random nature of the radiation emission from the patient, more than one radiation event may arrive at the detector within the finite response time of the detector. This phenomenon is known as pulse pile-up; when it occurs, it is not possible accurately to determine either the time of arrival of the event nor the energy of the radiation event as one event is corrupted by the arrival of the next event. Thus, when pulse pile-up occurs, it is not possible to validly classify any of the events as coincidence events and the data must be discarded. Excessively protracted measurement time or tracer dose may also become necessary if count rate is too low owing to pulse pile-up.
Furthermore, owing to the design of the detector units of a PET machine, pulse pile-up events can lead to the mis-assignment of the position of detection within the detector crystal. The specific position of the radiation/detector interaction can be determined using a position sensitive photo-multiplier tube (PMT) and ‘Anger’ logic based on the output of the four anodes of the PMT. When a pile-up event occurs, the weighting of the radiation event within the crystal array is incorrect, so an interaction that occurred in one crystal block may be miss-assigned to a neighboring crystal in the detector block.